More reliable beacon ingestion under high concurrency

We have hardened the ingestion path for Video Data beacons. The ingestion layer now handles concurrent message batches and partially-populated payloads more robustly. What changed:

1. Concurrency — events for the same view are processed in a deterministic order, improving consistency under heavy load.
2. Null-safe fields — events with optional fields (such as video_title or other player metadata) arriving as null or empty strings are now processed correctly.
3. No empty-string overwrites — empty strings in incoming events no longer overwrite valid existing values during the merge step.
4. Better diagnostics — additional structured logs make it easier for our team to investigate rare edge cases without redeploying.